Đáp án:
$137-17\vdots a\Rightarrow 120\vdots a$
$101-11\vdots a\Rightarrow 90\vdots a$
$67-7\vdots a\Rightarrow 60\vdots a$
$\Rightarrow a\in ƯC(120,90,60)$
mà $ƯCLN(120,90,60)=30$
$\Rightarrow a\in Ư(30)=\{1,2,3,5,6,10,15,30\}$
Vậy $a\in\{1,2,3,5,6,10,15,30\}$